Wafer producing method

ABSTRACT

A hexagonal single crystal wafer is produced from a hexagonal single crystal ingot. A separation start point is formed by setting a focal point of a laser beam inside the ingot at a predetermined depth from the upper surface of the ingot, which depth corresponds to a thickness of the wafer to be produced. The laser beam is applied to the upper surface of the ingot while relatively moving the focal point and the ingot to thereby form a modified layer parallel to the upper surface of the ingot and form cracks extending from the modified layer along a c-plane, thus forming a separation start point. The wafer is separated by immersing the ingot in water and then applying ultrasonic vibration to the ingot, thereby separating a plate-shaped member having a thickness corresponding to the thickness of the wafer from the ingot.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wafer producing method for slicing ahexagonal single crystal ingot to produce a wafer.

2. Description of the Related Art

Various devices such as ICs and LSIs are formed by forming a functionallayer on the front side of a wafer formed of silicon or the like andpartitioning this functional layer into a plurality of regions along aplurality of crossing division lines. The division lines of the waferare processed by a processing apparatus such as a cutting apparatus anda laser processing apparatus to thereby divide the wafer into aplurality of individual device chips corresponding to the respectivedevices. The device chips thus obtained are widely used in variousequipment such as mobile phones and personal computers. Further, powerdevices or optical devices such as LEDs and LDs are formed by forming afunctional layer on the front side of a wafer formed of a hexagonalsingle crystal such as SiC and GaN and partitioning this functionallayer into a plurality of regions along a plurality of crossing divisionlines.

In general, the wafer on which the devices are to be formed is producedby slicing an ingot with a wire saw. Both sides of the wafer obtainedabove are polished to a mirror finish (see Japanese Patent Laid-open No.2000-94221, for example). This wire saw is configured in such a mannerthat a single wire such as a piano wire having a diameter of about 100to 300 μm is wound around many grooves formed on usually two to fourguide rollers to form a plurality of cutting portions spaced in parallelwith a given pitch. The wire is operated to run in one direction oropposite directions, thereby slicing the ingot into a plurality ofwafers.

However, when the ingot is cut by the wire saw and both sides of eachwafer are polished to obtain the product, 70 to 80% of the ingot isdiscarded to cause a problem of poor economy. In particular, a hexagonalsingle crystal ingot of SiC or GaN, for example, has high Mohs hardnessand it is therefore difficult to cut this ingot with the wire saw.Accordingly, considerable time is required for cutting of the ingot,causing a reduction in productivity. That is, there is a problem inefficiently producing a wafer in this prior art.

A technique for solving this problem is described in Japanese PatentLaid-open No. 2013-49461. This technique includes the steps of settingthe focal point of a laser beam having a transmission wavelength to SiCinside a hexagonal single crystal ingot, next applying the laser beam tothe ingot as scanning the laser beam on the ingot to thereby form amodified layer and cracks in a separation plane inside the ingot, andnext applying an external force to the ingot to thereby break the ingotalong the separation plane where the modified layer and the cracks areformed, thus separating a wafer from the ingot. In this technique, thelaser beam is scanned spirally or linearly along the separation plane sothat a first application point of the laser beam and a secondapplication point of the laser beam nearest to the first applicationpoint have a predetermined positional relation with each other. As aresult, the modified layer and the cracks are formed at very highdensity in the separation plane of the ingot.

SUMMARY OF THE INVENTION

However, in the ingot cutting method described in Japanese PatentLaid-open No. 2013-49461 mentioned above, the laser beam is scannedspirally or linearly on the ingot. In the case of linearly scanning thelaser beam, the direction of scanning of the laser beam is notspecified. In the ingot cutting method described in Japanese PatentLaid-open No. 2013-49461, the pitch (spacing) between the firstapplication point and the second application point of the laser beam asmentioned above is set to 1 to 10 μm. This pitch corresponds to thepitch of the cracks extending from the modified layer along a c-planedefined in the ingot.

In this manner, the pitch of the application points of the laser beam tobe applied to the ingot is very small. Accordingly, regardless ofwhether the laser beam is scanned spirally or linearly, the laser beammust be applied with a very small pitch and the improvement inproductivity is not yet sufficient.

It is therefore an object of the present invention to provide a waferproducing method which can efficiently produce a wafer from an ingot.

In accordance with an aspect of the present invention, there is provideda wafer producing method for producing a hexagonal single crystal waferfrom a hexagonal single crystal ingot having a first surface, a secondsurface opposite to the first surface, a c-axis extending from the firstsurface to the second surface, and a c-plane perpendicular to thec-axis, the wafer producing method including: a separation start pointforming step of setting a focal point of a laser beam having atransmission wavelength to the ingot inside the ingot at a predetermineddepth from the first surface, which depth corresponds to a thickness ofthe wafer to be produced, and next applying the laser beam to the firstsurface as relatively moving the focal point and the ingot to therebyform a modified layer parallel to the first surface and cracks extendingfrom the modified layer along the c-plane, thus forming a separationstart point; and a wafer separating step of separating a plate-shapedmember having a thickness corresponding to the thickness of the waferfrom the ingot at the separation start point after performing theseparation start point forming step, thus producing the wafer from theingot. The separation start point forming step includes: a modifiedlayer forming step of relatively moving the focal point of the laserbeam in a first direction perpendicular to a second direction where thec-axis is inclined by an off angle with respect to a normal to the firstsurface and the off angle is formed between the first surface and thec-plane, thereby linearly forming the modified layer extending in thefirst direction; and an indexing step of relatively moving the focalpoint in the second direction to thereby index the focal point by apredetermined amount. The wafer separating step includes the steps ofimmersing the hexagonal single crystal ingot in water and then applyingultrasonic vibration to the ingot, thereby separating the plate-shapedmember from the ingot to produce the hexagonal single crystal wafer.

According to the wafer producing method of the present invention, eachmodified layer is formed at the predetermined depth from the firstsurface of the ingot and the cracks are formed on both sides of eachmodified layer so as to propagate along the c-plane. Accordingly, anyadjacent ones of the plural modified layers are connected togetherthrough the cracks formed therebetween, so that when the ingot isimmersed in water and ultrasonic vibration is then applied to the ingot,the plate-shaped member having the thickness corresponding to thethickness of the wafer can be easily separated from the ingot at theseparation start point, thus producing the hexagonal single crystalwafer from the ingot. Accordingly, the productivity can be sufficientlyimproved, and the amount of the ingot to be discarded can besufficiently reduced to about 30%.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and an appended claims with reference to theattached drawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a laser processing apparatus suitablefor use in performing a wafer producing method of the present invention;

FIG. 2 is a block diagram of a laser beam generating unit;

FIG. 3A is a perspective view of a hexagonal single crystal ingot;

FIG. 3B is an elevational view of the ingot shown in FIG. 3A;

FIG. 4 is a perspective view for illustrating a separation start pointforming step;

FIG. 5 is a plan view of the ingot shown in FIG. 3A;

FIG. 6 is a schematic sectional view for illustrating a modified layerforming step;

FIG. 7 is a schematic plan view for illustrating the modified layerforming step;

FIG. 8A is a schematic plan view for illustrating an indexing step;

FIG. 8B is a schematic plan view for illustrating an index amount;

FIG. 9; is a perspective view for illustrating a wafer separating step;

FIG. 10 is a perspective view of a hexagonal single crystal waferproduced from the ingot; and

FIGS. 11A to 11C are schematic sectional views for illustrating theseparation start point forming step according to a preferred embodimentof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be described indetail with reference to the drawings. Referring to FIG. 1, there isshown a perspective view of a laser processing apparatus 2 suitable foruse in performing a wafer producing method of the present invention. Thelaser processing apparatus 2 includes a stationary base 4 and a firstslide block 6 mounted on the stationary base 4 so as to be movable inthe X direction. The first slide block 6 is moved in a feedingdirection, or in the X direction along a pair of guide rails 14 by afeeding mechanism 12 composed of a ball screw 8 and a pulse motor 10.

A second slide block 16 is mounted on the first slide block 6 so as tobe movable in the Y direction. The second slide block 16 is moved in anindexing direction, or in the Y direction along a pair of guide rails 24by an indexing mechanism 22 composed of a ball screw 18 and a pulsemotor 20. A support table 26 is mounted on the second slide block 16.The support table 26 is movable in the X direction and the Y directionby the feeding mechanism 12 and the indexing mechanism 22 and alsorotatable by a motor stored in the second slide block 16.

A column 28 is provided on the stationary base 4 so as to project upwardtherefrom. A laser beam applying mechanism (laser beam applying means)30 is mounted on the column 28. The laser beam applying mechanism 30 iscomposed of a casing 32, a laser beam generating unit 34 (see FIG. 2)stored in the casing 32, and focusing means (laser head) 36 mounted onthe front end of the casing 32. An imaging unit 38 having a microscopeand a camera is also mounted on the front end of the casing 32 so as tobe aligned with the focusing means 36 in the X direction.

As shown in FIG. 2, the laser beam generating unit 34 includes a laseroscillator 40 for generating a pulsed laser beam such as YAG laser andYVO4 laser, repetition frequency setting means 42 for setting therepetition frequency of the pulsed laser beam to be generated from thelaser oscillator 40, pulse width adjusting means 44 for adjusting thepulse width of the pulsed laser beam to be generated from the laseroscillator 40, and power adjusting means 46 for adjusting the power ofthe pulsed laser generated from the laser oscillator 40. Althoughespecially not shown, the laser oscillator 40 has a Brewster window, sothat the laser beam generated from the laser oscillator 40 is a laserbeam of linearly polarized light. After the power of the pulsed laserbeam is adjusted to a predetermined power by the power adjusting means46 of the laser beam generating unit 34, the pulsed laser beam isreflected by a mirror 48 included in the focusing means 36 and nextfocused by a focusing lens 50 included in the focusing means 36. Thefocusing lens 50 is positioned so that the pulsed laser beam is focusedinside a hexagonal single crystal ingot 11 as a workpiece fixed to thesupport table 26.

Referring to FIG. 3A, there is shown a perspective view of the hexagonalsingle crystal ingot 11 as a workpiece to be processed. FIG. 3B is anelevational view of the hexagonal single crystal ingot 11 shown in FIG.3A. The hexagonal single crystal ingot (which will be hereinafterreferred to also simply as ingot) 11 is selected from a SiC singlecrystal ingot or a GaN single crystal ingot. The ingot 11 has a firstsurface (upper surface) 11 a and a second surface (lower surface) 11 bopposite to the first surface 11 a. The first surface 11 a of the ingot11 is preliminarily polished to a mirror finish because the laser beamis applied to the first surface 11 a.

The ingot 11 has a first orientation flat 13 and a second orientationflat 15 perpendicular to the first orientation flat 13. The length ofthe first orientation flat 13 is set greater than the length of thesecond orientation flat 15. The ingot 11 has a c-axis 19 inclined by anoff angle α toward the second orientation flat 15 with respect to anormal 17 to the upper surface 11 a and also has a c-plane 21perpendicular to the c-axis 19. The c-plane 21 is inclined by the offangle α with respect to the upper surface 11 a. In general, in thehexagonal single crystal ingot 11, the direction perpendicular to thedirection of extension of the shorter second orientation flat 15 is thedirection of inclination of the c-axis 19. The c-plane 21 is set in theingot 11 innumerably at the molecular level of the ingot 11. In thispreferred embodiment, the off angle α is set to 4°. However, the offangle α is not limited to 4° in the present invention. For example, theoff angle α may be freely set in the range of 1° to 6° in manufacturingthe ingot 11.

Referring again to FIG. 1, a column 52 is fixed to the left side of thestationary base 4. The column 52 is formed with a vertically elongatedopening 53, and a pressing mechanism 54 is vertically movably mounted tothe column 52 so as to project from the opening 53.

As shown in FIG. 4, the ingot 11 is fixed to the upper surface of thesupport table 26 by using a wax or adhesive in the condition where thesecond orientation flat 15 of the ingot 11 becomes parallel to the Xdirection. In other words, as shown in FIG. 5, the direction offormation of the off angle α is shown by an arrow Y1. That is, thedirection of the arrow Y1 is the direction where the intersection 19 abetween the c-axis 19 and the upper surface 11 a of the ingot 11 ispresent with respect to the normal 17 to the upper surface 11 a.Further, the direction perpendicular to the direction of the arrow Y1 isshown by an arrow A. Then, the ingot 11 is fixed to the support table 26in the condition where the direction of the arrow A becomes parallel tothe X direction.

Accordingly, the laser beam is scanned in the direction of the arrow Aperpendicular to the direction of the arrow Y1, or the direction offormation of the off angle α. In other words, the direction of the arrowA perpendicular to the direction of the arrow Y1 where the off angle αis formed is defined as the feeding direction of the support table 26.

In the wafer producing method of the present invention, it is importantthat the scanning direction of the laser beam to be applied from thefocusing means 36 is set to the direction of the arrow A perpendicularto the direction of the arrow Y1 where the off angle α of the ingot 11is formed. That is, it was found that by setting the scanning directionof the laser beam to the direction of the arrow A as mentioned above inthe wafer producing method of the present invention, cracks propagatingfrom a modified layer formed inside the ingot 11 by the laser beamextend very long along the c-plane 21.

In performing the wafer producing method according to this preferredembodiment, a separation start point forming step is performed in such amanner that the focal point of the laser beam having a transmissionwavelength (e.g., 1064 nm) to the hexagonal single crystal ingot 11fixed to the support table 26 is set inside the ingot 11 at apredetermined depth from the first surface (upper surface) 11 a, whichdepth corresponds to the thickness of a wafer to be produced, and thelaser beam is next applied to the upper surface 11 a as relativelymoving the focal point and the ingot 11 to thereby form a modified layer23 parallel to the upper surface 11 a and cracks 25 propagating from themodified layer 23 along the c-plane 21, thus forming a separation startpoint (separation plane) where the modified layer 23 and the cracks 25are formed.

This separation start point forming step includes a modified layerforming step of relatively moving the focal point of the laser beam inthe direction of the arrow A perpendicular to the direction of the arrowY1 where the c-axis 19 is inclined by the off angle α with respect tothe normal 17 to the upper surface 11 a and the off angle α is formedbetween the c-plane 21 and the upper surface 11 a, thereby forming themodified layer 23 inside the ingot 11 and the cracks 25 propagating fromthe modified layer 23 along the c-plane 21, and also includes anindexing step of relatively moving the focal point in the direction offormation of the off angle α, i.e., in the Y direction to thereby indexthe focal point by a predetermined amount as shown in FIG. 7 and FIGS.8A and 8B.

As shown in FIGS. 6 and 7, the modified layer 23 is linearly formed soas to extend in the X direction, so that the cracks 25 propagate fromthe modified layer 23 in opposite directions along the c-plane 21. Inthe wafer producing method according to this preferred embodiment, theseparation start point forming step further includes an index amountsetting step of measuring the width of the cracks 25 formed on one sideof the modified layer 23 along the c-plane 21 and then setting the indexamount of the focal point according to the width measured above. Morespecifically, letting W1 denote the width of the cracks 25 formed on oneside of the modified layer 23 so as to propagate from the modified layer23 along the c-plane 21, the index amount W2 of the focal point is setin the range of W1 to 2W1.

For example, the separation start point forming step is performed underthe following laser processing conditions.

Light source: Nd:YAG pulsed laser

Wavelength: 1064 nm

Repetition frequency: 80 kHz

Average power: 3.2 W

Pulse width: 4 ns

Spot diameter: 10 μm

Numerical aperture (NA) of the focusing lens: 0.45

Index amount: 400 μm

In the laser processing conditions mentioned above, the width W1 of thecracks 25 propagating from the modified layer 23 along the C-plane 21 inone direction as viewed in FIG. 6 is set to about 250 μm, and the indexamount W2 is set to 400 μm. However, the average power of the laser beamis not limited to 3.2 W. When the average power of the laser beam wasset to 2 to 4.5 W, good results were obtained in the preferredembodiment. In the case that the average power was set to 2 W, the widthW1 of the cracks 25 was about 100 μm. In the case that the average powerwas set to 4.5 W, the width W1 of the cracks 25 was about 350 μm.

In the case that the average power is less than 2 W or greater than 4.5W, the modified layer 23 cannot be well formed inside the ingot 11.Accordingly, the average power of the laser beam to be applied ispreferably set in the range of 2 to 4.5 W. For example, the averagepower of the laser beam to be applied to the ingot 11 was set to 3.2 Win this preferred embodiment. As shown in FIG. 6, the depth D1 of thefocal point from the upper surface 11 a in forming the modified layer 23was set to 500 μm.

Referring to FIG. 8A, there is shown a schematic plan view forillustrating the scanning direction of the laser beam. The separationstart point forming step is performed on a forward path X1 and abackward path X2 as shown in FIG. 8A. That is, the modified layer 23 isformed in the hexagonal single crystal ingot 11 on the forward path X1.Thereafter, the focal point of the laser beam is indexed by thepredetermined amount. Thereafter, the modified layer 23 is formed againin the ingot 11 on the backward path X2.

Further, in the case that the index amount of the focal point of thelaser beam is set in the range of W to 2W where W is the width of thecracks 25 formed on one side of the modified layer 23 along the c-plane21, the index amount of the focal point is preferably set to W or lessuntil the modified layer 23 is first formed after setting the focalpoint of the laser beam inside the ingot 11.

For example, in the case that the index amount of the focal point of thelaser beam is 400 μm, the index amount is set to 200 μm until themodified layer 23 is first formed inside the ingot 11, and the laserbeam is scanned plural times with this index amount of 200 μm as shownin FIG. 8B. That is, a first part of the plural scanning paths of thelaser beam is idle, and when it is determined that the modified layer 23has been first formed inside the ingot 11, the index amount is set to400 μm and the modified layer 23 is then formed inside the ingot 11.

In this manner, the focal point of the laser beam is sequentiallyindexed to form a plurality of modified layers 23 at the depth D1 in thewhole area of the ingot 11 and also form the cracks 25 extending fromeach modified layer 23 along the c-plane 21. Thereafter, a waferseparating step is performed in such a manner that an external force isapplied to the ingot 11 to thereby separate a plate-shaped member havinga thickness corresponding to the thickness of the wafer to be formedfrom the ingot 11 at the separation start point composed of the modifiedlayers 23 and the cracks 25, thus producing a hexagonal single crystalwafer 27 shown in FIG. 10.

This wafer separating step is performed in the following manner. Asshown in FIG. 9, an ultrasonic vibration generating apparatus 62 is setin a water tank 60, and the hexagonal single crystal ingot 11 in whichthe separation start point has been formed by the previous step ismounted on the ultrasonic vibration generating apparatus 62. Thereafter,the water tank 60 is filled with pure water 64 to immerse the hexagonalsingle crystal ingot 11 in the pure water 64. Thereafter, a voltage isapplied to the ultrasonic vibration generating apparatus 62 to therebygenerate ultrasonic vibration having a frequency of 40 kHz, for example.

This ultrasonic vibration is transmitted to the hexagonal single crystalingot 11, thereby separating the plate-shaped member having a thicknesscorresponding to the thickness of the wafer from the ingot 11 at theseparation start point formed inside the ingot 11. Accordingly, ahexagonal single crystal wafer 27 as shown in FIG. 10 can be producedfrom the hexagonal single crystal ingot 11. While the pure water 64 iscontained in the water tank 60 in this preferred embodiment, any othertypes of containers are usable.

As the ultrasonic vibration generating apparatus 62 for generatingultrasonic vibration, an AS ultrasonic cleaner US-2R provided by AS ONECorporation may be used, for example. In this case, the ultrasonicvibration generating apparatus 62 can generate ultrasonic vibrationhaving a frequency of 40 kHz and a power of 80 W. The ultrasonicvibration to be applied in the wafer separating step preferably has afrequency of 30 to 50 kHz, more preferably 35 to 45 kHz.

The separation start point forming step according to this preferredembodiment will now be described in more detail with reference to FIGS.11A to 11C. As shown in FIG. 11A, the focal point P of a laser beam LBis set inside the ingot 11 at the depth corresponding to the thicknessof the wafer to be produced, and the laser beam LB is next applied tothe upper surface 11 a of the ingot 11. As a result, the modified layer23 is formed at the focal point P and the cracks 25 are formed aroundthe modified layer 23 so as to radially extend therefrom along thec-plane.

That is, when the laser beam LB is applied once, the modified layer 23is formed at the focal point P and the cracks 25 propagating from themodified layer 23 are formed around the modified layer 23 in the c-planeas shown in FIG. 11B. In the separation start point forming stepaccording to this preferred embodiment, the focal point P is set at themodified layer 23 already formed as mentioned above, and the laser beamis next applied to the upper surface 11 a of the ingot 11. This laserbeam applying step is repeated plural times. In this laser beam applyingstep, the laser beam is scanned from one end of the ingot 11 to theother end thereof and this scanning operation is repeated plural timesat the same repetition frequency and the same feed speed as those in theprevious cycle.

For example, about 5 to 7 cycles of the laser beam scanning operationare performed and the laser beam is applied to the ingot 11 in thecondition where the focal point of the laser beam is set at eachmodified layer 23 previously formed. As a result, the cracks 25 areseparated from each modified layer 23 and connected together on theopposite sides of each modified layer 23 as shown in FIG. 11C. Since thecracks 25 are separated from each modified layer 23 and connectedtogether as described above, the subsequent separating step can beperformed very easily. That is, the plate-shaped member having athickness corresponding to the thickness of the wafer can be easilyseparated from the hexagonal single crystal ingot 11 at the separationstart point, thus easily producing the hexagonal single crystal wafer 27from the ingot 11.

The present invention is not limited to the details of the abovedescribed preferred embodiment. The scope of the invention is defined bythe appended claims and all changes and modifications as fall within theequivalence of the scope of the claims are therefore to be embraced bythe invention.

What is claimed is:
 1. A wafer producing method for producing a hexagonal single crystal wafer from a hexagonal single crystal ingot having a first surface, a second surface opposite to the first surface, a c-axis extending from the first surface to the second surface, and a c-plane perpendicular to the c-axis, the wafer producing method comprising: a separation start point forming step of setting a focal point of a laser beam having a transmission wavelength to the ingot inside the ingot at a predetermined depth from the first surface, which depth corresponds to a thickness of the wafer to be produced, and next applying the laser beam to the first surface as relatively moving the focal point and the ingot to thereby form a modified layer parallel to the first surface and cracks extending from the modified layer along the c-plane, thus forming a separation start point; and a wafer separating step of separating a plate-shaped member having a thickness corresponding to the thickness of the wafer from the ingot at the separation start point after performing the separation start point forming step, thus producing the wafer from the ingot; the separation start point forming step including a modified layer forming step of relatively moving the focal point of the laser beam in a first direction perpendicular to a second direction where the c-axis is inclined by an off angle with respect to a normal to the first surface and the off angle is formed between the first surface and the c-plane, thereby linearly forming the modified layer extending in the first direction, and an indexing step of relatively moving the focal point in the second direction to thereby index the focal point by a predetermined amount; the wafer separating step including the steps of immersing the hexagonal single crystal ingot in water and then applying ultrasonic vibration to the ingot, thereby separating the plate-shaped member from the ingot to produce the hexagonal single crystal wafer.
 2. The wafer producing method according to claim 1, wherein in the separation start point forming step, the laser beam is applied to the ingot plural times in a condition where the focal point of the laser beam is set at the modified layer previously formed, thereby separating the cracks from the modified layer.
 3. The wafer producing method according to claim 1 wherein the hexagonal single crystal ingot is selected from an SiC single crystal ingot and a GaN single crystal ingot. 